Cnidarians have large surface areas available for colonization by microbial organisms, which serve a multitude of functions in the environment. However, relatively few studies have been conducted on scyphozoan-associated microbial communities. Blooms of scyphozoan species are common worldwide and can have numerous deleterious consequences on the marine ecosystem. Four scyphozoan species, Aurelia coerulea, Cyanea nozakii, Nemopilema nomurai, and Rhopilema esculentum, form large blooms in Chinese seas. In this study, we analyzed the bacterial communities associated with these four jellyfish based on 16S rRNA gene sequencing. We found that the bacterial communities associated with each scyphozoan species were significantly different from each other and from those of the surrounding seawater. There were no significant differences between the bacterial communities associated with different body parts of the four scyphozoan jellyfish. Core bacteria in various compartments of the four scyphozoan taxa comprised 57 OTUs (Operational Taxonomic Units), dominated by genera Mycoplasma, Vibrio, Ralstonia, Tenacibaculum, Shingomonas and Phyllobacterium. FAPROTAX function prediction revealed that jellyfish could influence microbially mediated biogeochemical cycles, compound degradation and transmit pathogens in regions where they proliferate. Finally, Six genera of potentially pathogenic bacteria associated with the scyphozoans were detected: Vibrio, Mycoplasma, Ralstonia, Tenacibaculum, Nautella, and Acinetobacter. Our study suggests that blooms of these four common scyphozoans may cause jellyfish species-specific impacts on element cycling in marine ecosystems, and serve as vectors of pathogenic bacteria to threaten other marine organisms and human health.

The moon jellyfish Aurelia coerulea is a nuisance species around the world, and is considered highly tolerant of a wide range of environmental conditions. The recruitment success during the early life stages of scyphomedusae can have a major effect on the abundance of the adult medusa population and can contribute to jellyfish blooms. The environmental stress factors of elevated copper concentrations and reduced salinity often act simultaneously during the summer time. In this study, we investigated the combined effects of three concentrations of copper (0, 10 and 25 μg L −1) and reduced salinity (from ambient 31 psu to 22 psu) on the early life stages of A. coerulea. We found that the swimming speed of the planula larvae of A. coerulea was significantly affected by the copper concentration. Planula larvae of A. coerulea from the highest copper concentration (25 μg L −1) moved slower at than at lower copper concentrations. The results showed significant interactive effects between copper concentrations and salinity on the settlement of A. coerulea planulae. High copper concentration (25 μg L −1) and reduced salinity (22 psu) significantly increased the settlement of A. coerulea planulae. Additionally, copper concentration had a significant effect on the asexual reproduction rate of A. coerulea polyps, which was significantly higher at the moderate copper concentration (10 μg L −1). These results suggest that the current concentration of copper was not a stress factor for the early life stages of A. coerulea and may potentially stimulate population increases of A. coerulea in Chinese coastal waters.